1. Field of the Invention
The present invention relates generally to the production of shaped, tempered sheets of heat-softenable material and, more particularly, to an improved method of and apparatus for shaping and heat treating relatively thin glass sheets.
Shaped glass sheets are widely used as side windows and roof panels in vehicles such as automobiles or the like and, to be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window openings into which the glass windows or panels are installed. It is also important that the side windows or panels meet stringent optical requirements and that the windows or panels be free of optical defects that would tend to interfere with the clear viewing therethrough in their viewing area. During fabrication, glass sheets intended for use as shaped windows or panels in vehicles are subjected to thermal treatment to temper the glass for strengthening the same and increasing the resistance of the shaped windows to damage resulting from impact.
The commercial production of shaped glass sheets for such purposes commonly includes heating flat sheets to the softening point of the glass, shaping the heated sheets to a desired curvature and then cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. To promote efficient and large scale production, discrete glass sheets are conventionally heated, bent and cooled while being moved continuously along a fixed path and successively through a heating section, a roll forming section, and a cooling section. To achieve satisfactory temper, the temperature of the glass sheet must be above a predetermined minimum level so as to maintain the core or interior thereof above a deformation temperature upon being exposed initially to cooling medium at the cooling section. The residual heat remaining in glass sheets of previous commercial thicknesses, such as those having nominal thicknesses ranging from 4.5 millimeters to 6 millimeters, is generally sufficient after shaping for immediate advancement to the tempering area and exposure to the cooling medium. Thus, the heat initially imparted to a relatively thick glass sheet to bring it to proper temperature for shaping can also be utilized in the final heat treating operation.
However, within the last several years, considerable emphasis has been placed on the use of thinner and thinner glass sheets for automobile side windows as a means of reducing overall weight of the autos as a means to obtain better fuel mileage. This has posed problems in shaping and tempering, due to the lesser ability of the thinner sheets to retain heat and the aforementioned conventional process of bending and treating glass sheets does not lend itself to the processing of these relatively thin sheets, such as those having nominal thicknesses ranging from less than 3 millimeters to 4 millimeters (90 mils to 160 mils). As the thickness of the glass decreases, the rate of heat loss increases and the heat initially imparted to such thin sheets is quickly dissipated upon leaving the heating atmosphere of the furnace and during the relatively cool bending cycle. Attempts to solve these problems by initially overheating the thin glass sheets have not been successful because of the consequent loss of control of the glass shaping process and the degradation of the surface quality of the finished glass as a result of heat stains, roll ripple distortion, and the imposition of roll marks in the surface of the heat-softened glass sheet.
Consequently, roll forming has ben developed as a technique for shaping and tempering glass sheets at a high production rate. One of the benefits of the roll forming process is the rapid removal of each individual glass sheet from the heating section or furnace through the shaping section and into the quenching section. In the roll forming method, glass sheets are conveyed without stopping through heating, shaping, and tempering sections along high speed glass sheet conveyor means to drastically reduce the time needed to traverse the distance between the exit of the heating section or furnace to the tempering or quenching section to a minimum, preferably under 5 seconds. Under such circumstances, thin glass sheets can be tempered by quenching without imparting such a high initial temperature at the furnace that shape control and control of surface quality is lost as a consequence of insuring that the temperature at the core of each glass sheet does not cool to below the minimum temperature needed on arrival at the quenching section to assure adequate temper. However, moving glass sheets engaged simultaneously between upper and lower rotating forming rolls tend to skew and/or to develop rub marks in the glass surfaces unless the operation of the forming rolls is carefully controlled.
In roll forming as practiced in the prior art, either a continuous glass ribbon or a series of discrete glass sheets is heated to or above the deformation temperature of the glass and passed in a continuous motion through one or more shaping stations where the glass sheets pass between upper and lower rotating forming rolls of complementary shape to change the glass sheets from a flat configuration to a shaped configuration. Shaping individual glass sheets by roll forming, particularly those of non-rectangular shape having one or both longitudinal side edges extending obliquely of the path of glass sheet movement, is more difficult to perform than roll forming a continuous ribbon, because individual glass sheets having leading edges as well as side edges that are prone to be distorted by a high speed shaping operation, whereas only the side edges of a continuous ribbon are more prone to distortion than the main body of the glass.
The history of prior art attempts to shape glass sheets continuously without causing the glass sheets to stop for the shaping step so as to obtain as high a production rate of shaped glass sheets as possible and the problems associated with shaping thin glass sheets will be understood better in the light of a description of the prior art that follows.
2. Description of the Prior Art
Many patents have been issued on roll forming.
Drake U.S. Pat. No. 2,348,887 moves heated glass sheets between a pair of aligned pressure rolls 32 and 33 of cylindrical configuration which force the bottom surfaces of the glass sheets to ride over a series of spaced bending rolls 31 of cylindrical configuration mounted for rotation along spaced lines that extend transversely of a curved pth corresponding to the shape desired for the bent glass sheets. The shapes imparted to the moving glass sheets are limited to cylindrical curvatures of uniform radius about an axis transverse to the path of glass movement. Simultaneous rolling contact of the pressure rolls against the upper and lower surfaces causes surface marring.
Ritter et al U.S. Pat. No. 3,881,906 sags heated glass sheets to intermediate shapes of progressively increasing curvature transverse to their path of movement by conveying said heated glass sheets on successive, contoured, rotating, conveyor rolls of increasing transverse curvature en route to a shaping station. The entire weight of a transversely extending leading element of the glass is borne entirely along the side edge portions of the glass as it transfers from one contoured forming roll to the next. Consequently, the lateral edges kink away from the overall curvature desired and it is necessary to stop each partially shaped glass sheet at a shaping station where its shaping is corrected by the inertia gravity method which involves the use of a shaping mold that moves in an upward vertical direction transverse to both the glass movement path and the axes of rotation of the contoured rolls to engage the glass sheet margin while the glass sheet forward movement is stopped.
U.S. Pat. Nos. 3,545,951, Bezombes 3,801,298 and 3,832,153 and Hoff et al. 3,831,239 shape moving glass sheets between shaped conveyor rolls that support the lower surface of moving heat-softened glass sheets and a movable upper shaping member of complementary configuration. The apparatus of these patents provides a family of simple curves about a single axis transverse to the path of glass sheet movement. These patents require the shaped conveyor rolls to rotate between different orientations from a flat glass supporting position to a shaped glass supporting position. The change in orientations must be correlated with glass sheet movement to obtain desired results. The simultaneous contact of rollers against the upper and lower glass sheet surfaces marks the glass.
U.S. Pat. Nos. Frank 3,701,644; 3,856,499; 3,871,855; 3,891,420; 3,929,441; 3,934,996; 3,992,181 and 4,043,783, and Knapp 3,869,269 disclose roll forming apparatus capable of shaping a succession of discrete moving glass sheets to either simple shapes provided with one component of shape about either an axis extending longitudinally of the path of glass sheet movement or about an axis extending transversely thereof or compound shapes involving various combinations of two components conforming to said simple shapes. In addition, the roll forming apparatus of this group of patents is capable of shaping glass sheets to either simple or compound shapes involving non-uniform radii of curvature.
This last group of patents provides different inventions incorporated in the most sophisticated system for shaping continuously moving glass sheets to various shapes at the highest rates of production attained prior to the present invention. However, even though this last group of patents provided the greatest variety of simple and compound shapes for glass sheets ever attained, the apparatus comprised movable parts whose movement between spaced apart positions on opposite sides of a path of movement provided by conveyor rolls for glass sheets and glass engaging positions where the rotating forming rolls engage the opposite major surfaces of the sheets to one side of said conveyor rolls had to be correlated with the glass sheet movement between the movable parts. This correlation required constant monitoring and frequent adjustment of moving parts in order to avoid surface marring and/or skewing of the glass sheets. In addition, it was necessary to spend considerable time for set up and adjustment of the apparatus when production patterns were changed to insure that the movements of the rotating shaping rolls toward and away from one another correlate properly with the movement of discrete glass sheets therebetween. Furthermore, simultaneous engagement of forming rolls against portions of the upper and lower glass sheet surfaces causes some residual light surface marking despite the precautions that are taken to reduce the severity of such surface marking.
U.S. Pat. No. 3,905,794 to Revells et al. discloses shaped conveyor rolls that rotate in unison from a flat glass supporting position to a rotated position where their central portions are lowered as a shaping mold lifts a stopped glass sheet into engagement with a complementary shaped upper shaping mold. This apparatus precludes continuous movement of the glass sheets through the shaping station en route to the cooling station, thereby limiting the maximum rate of production and the maximum temper possible.
U.S. Pat. No. 4,015,968 to Revells et al., U.S. Pat. No. 4,047,919 and U.S. Pat. No. 4,116,662 to Revells disclose different embodiments of shaped conveyor rolls used in conjunction with shaping molds that engage a glass sheet conveyed on said rolls to a glass sheet shaping station. Each of these patented apparatus stops the forward movement of the glass sheet at the shaping station to lift the glass sheet on the lower shaping mold. Such stopping increases the time needed to transport the hot glass sheet from the furnace exit to the entrance of the quenching station.
U.S. Pat. No. 4,123,246 to Johnson et al. discloses shaping glass sheets by gradually transferring a succession of glass sheets from an obliquely disposed series of stationary, cylindrically shaped conveyor rolls onto alternately disposed, shaped, rotating, stationary forming rolls and thence onto a succession of stationary, shaped, rotating, consecutive forming rolls en route to a cooling station without stopping movement of the glass sheets undergoing such treatment.
None of the patents disclose shaping a glass sheet by transferring it in the upstream portion of a shaping station from stationary, horizontally aligned, longitudinally spaced, cylindrically shaped, rotating conveyor rolls by lifting the moving glass sheets onto a first series of alternate, rotating, shaped, upwardly and rectilinearly translatable forming rolls and thence in the downstream portion of the shaping station onto a second series of consecutive, shaped, upwardly and rectilinearly translatable shaping rolls that move upwardly in unison with the alternate forming rolls of the first series in the upstream portion of the shaping station without engaging the upper surfaces of the sheets with a solid member.